Active matrix liquid crystal display devices are commercially available and used in, for example, portable terminals, liquid crystal televisions, projectors, and computers because of their high display quality. In active matrix liquid crystal display devices, a TFT (thin film transistor), a MIM (metal-insulator-metal), or the like is used in each pixel and a high voltage holding ratio is important for liquid crystal compounds or liquid crystal compositions used in active matrix liquid crystal display devices. Liquid crystal display devices obtained by combination with a VA (vertical alignment) mode, an IPS (in-plane switching) mode, or an OCB (optically compensated bend or optically compensated birefringence) mode have been proposed to achieve good visual properties. Furthermore, ECB (electrically controlled birefringence) mode reflective liquid crystal display devices have been proposed to achieve brighter display. At present, novel liquid crystal compounds or liquid crystal compositions are being proposed for such liquid crystal display devices.
A fringe field switching (FFS) mode liquid crystal display, which is one of IPS mode liquid crystal displays having high quality and good visual properties, is being widely used as a liquid crystal display for smart phones (refer to PTL 1 and PTL 2). An FFS mode has been introduced to address the low aperture ratio and low transmittance of an IPS mode. In an FFS mode, a material containing a p-type liquid crystal composition having a positive dielectric anisotropy is widely used because the voltage can be easily decreased. Since most of the application areas of the FFS mode are portable terminals, there is a high demand for lower power consumption and thus liquid crystal device manufacturers have been actively making development efforts such as adoption of an array that uses IGZO.
The transmittance can also be improved by changing a liquid crystal material from a currently used p-type material to an n-type material having a negative dielectric anisotropy (refer to PTL 3). The reason for this is as follows. In the FFS mode, a completely parallel electric field is not generated unlike the IPS mode. When a p-type material is used, the major axis of liquid crystal molecules located near a pixel electrode is inclined along the fringing field, which degrades the transmittance. In contrast, when an n-type liquid crystal composition is used, the influence of the fringing field is only on the rotation of the liquid crystal molecules about the major axis of the liquid crystal molecules because the polarization direction of the n-type liquid crystal composition is a minor-axis direction of the molecules. As a result, the parallel arrangement of the molecule major axes is maintained, and thus a decrease in the transmittance does not occur.
Although n-type liquid crystal compositions are typical liquid crystal compositions for a VA mode, the VA mode and the FFS mode are different from each other in terms of alignment direction, electric field direction, and required optical properties. Furthermore, FFS mode liquid crystal display devices have a distinctive feature in terms of electrode structure as described below. That is, both substrates include an electrode in the VA mode whereas only an array substrate includes an electrode in the FFS mode. Therefore, there is no knowledge about problems such as image sticking and drop marks, which makes it difficult to predict the improvement from the related art. Accordingly, it is difficult to provide a liquid crystal display device having the level of high performance required today by simply using a liquid crystal composition for a VA mode, and therefore an n-type liquid crystal composition optimized for an FFS mode is desired.